首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   649827篇
  免费   75345篇
  国内免费   336篇
  2016年   7092篇
  2015年   9813篇
  2014年   11447篇
  2013年   16808篇
  2012年   18805篇
  2011年   18967篇
  2010年   12771篇
  2009年   11985篇
  2008年   17106篇
  2007年   17522篇
  2006年   16767篇
  2005年   16160篇
  2004年   15867篇
  2003年   15657篇
  2002年   15121篇
  2001年   27384篇
  2000年   27613篇
  1999年   22360篇
  1998年   8150篇
  1997年   8451篇
  1996年   8236篇
  1995年   7784篇
  1994年   7957篇
  1993年   7793篇
  1992年   19389篇
  1991年   18612篇
  1990年   18491篇
  1989年   18360篇
  1988年   16962篇
  1987年   16397篇
  1986年   15183篇
  1985年   15373篇
  1984年   12680篇
  1983年   11254篇
  1982年   8757篇
  1981年   8016篇
  1980年   7554篇
  1979年   12581篇
  1978年   9894篇
  1977年   9111篇
  1976年   8776篇
  1975年   9471篇
  1974年   10176篇
  1973年   9990篇
  1972年   9200篇
  1971年   8199篇
  1970年   7259篇
  1969年   6994篇
  1968年   6315篇
  1967年   5570篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Kinetics of thermal inactivation of glyceraldehyde-3-phosphate dehydrogenases of mung beans and rabbit muscle have been studied under different pH conditions in the absence and presence of various concentrations of NAD+ and NADH. The data have been discussed with respect to the effect of the coenzymes on the quaternary structure symmetry of the two enzymes and their binding isotherms. Both the (homo-tetrameric) apo-enzymes exhibit biphasic kinetics of thermal inactivation, characteristic of C2 symmetry, at lower pH values and a single exponential decay of enzyme activity, characteristic of D2 symmetry, at higher pHs. In each case, NAD+ has no effect on the biphasic kinetic pattern of thermal inactivation at lower pH values, but NADH brings about a change to single exponential decay. At higher pH values, NADH does not affect the kinetic pattern (single exponential decay) of any enzyme, but NAD+ alters it to biphasic kinetics in each case. The data suggest that NAD+ and NADH have higher affinity for the C2 and D2 symmetry conformation, respectively. With mung beans enzyme, the effect of NAD+ on the two rate constants of biphasic inactivation at pH 7.3 is consistent with a Kdiss equal to 110 microM. The NAD(+)-dependent changes in the kinetic pattern of thermal inactivation of this enzyme at pH 8.6 suggest a positive cooperativity in the coenzyme binding (nH = 3.0). In the binding of NADH to the mung beans enzyme, a weak positive cooperativity is observed at pH 7.3.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
62.
63.
Bivariate flow karyotypes of chromosomes from sheep, cattle and pig lymphocytes and from a cattle-mouse somatic cell hybrid line were obtained using a dual laser fluorescence-activated cell sorter (FACS). Pig chromosomes were resolved into 19-20 peaks, indicating that most, if not all, pig chromosomes could be separated by this technique. Sheep chromosomes showed incomplete separation but three clear peaks, presumably representing the three large metacentric chromosomes, plus five other clusters were obtained. Cattle chromosomes showed poor separation but about four peaks could be distinguished, indicating that certain chromosomes could be sorted in this species. The use of cattle-mouse hybrids may enable other individual cattle chromosomes to be obtained. It is concluded that FACS separation will be a useful additional tool for gene mapping.  相似文献   
64.
In this paper the electrostatic interactions between membrane-embedded ion-pumps and their consequences for the kinetics of pump-mediated transport processes have been examined. We show that the time course of an intrinsically monomolecular transport reaction can become distinctly nonexponential, if the reaction is associated with charge translocation and takes place in an aggregate of pump molecules. First we consider the electrostatic coupling of a single dimer of ion-pumps embedded in the membrane. Then we apply the treatment to the kinetic analysis of light-driven proton transport by bacteriorhodopsin which forms two-dimensional hexagonal lattices. Finally, for the case of nonordered molecules, we also consider a model in which the pumps are randomly distributed over the nodes of a lattice. Here the average distance is equal to that deduced experimentally and the elemental size of the lattice is the effective diameter of one single pump. This latter model is applied to an aggregate of membrane-embedded Na, K- and Ca-pumps. In all these cases the electrostatic potential considered is the exact solution calculated from the method of electrical images for a plane membrane of finite thickness immersed in an infinite aqueous solution environment. The distributions of charges (ions or charged binding sites) are considered homogeneous or discrete in the membrane and/or in the external solution. In the case of discrete distributions we compare the results from a mean field approximation and a stochastic simulation.  相似文献   
65.
66.
The transporter associated with the antigen processing 1 (TAP1) gene encodes a subunit for a transporter, presumed to be involved in the delivery of peptides across the endoplasmic reticulum membrane to class I molecules. We have generated mice with a disrupted TAP1 gene using embryonic stem cell technology. TAP1-deficient mice are defective in the stable assembly and intracellular transport of class I molecules and consequently show severely reduced levels of surface class I molecules. These properties are strikingly similar to those described for the TAP2 mutant cell line RMA-S. Cells from the TAP1-deficient mice are unable to present cytosolic antigens to class I-restricted cytotoxic T cells. As predicted from the near absence of class I surface expression, TAP1-deficient mice lack CD4-8+ T cells.  相似文献   
67.
A method of pH distribution measurements in agar nutrient media containing expanding bacterial populations is described. It is based on measuring pH microsamples taken at different points of the media. The sample volume was 10 microliters. A pH sensitive field effect transistor was used as a measuring electrode. Acidification was found to occur in glucose media, while alkalization occurred in the media containing peptone.  相似文献   
68.

1. 1.|An experiment was carried out to examine the effects of various levels of infra-red (i.r.) radiation on rectal temperature (RT) and respiration rate (RR) in New Zealand While rabbits.

2. 2.|A 4 × 3 × 6 factorial design was employed in which the factors were: four intensities of i.r. radiant heating of 0.0, 1.9, 2.1 and 2.4 MJ/m2/h, three replicates and six rabbits.

3. 3.|rectal temperature differed (P < 0.05) between treatments and were highest at the “high” level of i.r. radiation (1°C higher than for controls). At the “medium” and “low” levels of i.r. heating RTs were respectively 0.3 and 0.2°C higher than in controls.

4. 4.|At different levels of radiation RR were different (P < 0.05), with the highest (422.7 ± 218.1 breaths/min) at 2.4 MJ/m2/h i.r. radiant heating. This RR was almost 2.5 times that in controls, while at the “low” and “medium” i.r. levels RR values were respectively 1.5 and 2 times those of controls.

Author Keywords: Rabbit; thermoregulation; infra-red; higher critical temperature  相似文献   

69.
We have used a reconstitution assay to demonstrate that protein translocation activity can be recovered after microsomal vesicles derived from the rough endoplasmic reticulum have been partially solubilized with n-octyl-beta-glucopyranoside. Two independent approaches were used to establish conditions for partially solubilizing microsomal membranes. When the lipid bilayer was disrupted by detergents to the extent that the integrity of the lipid bilayer had been perturbed, membranes were inactive for translocation. However, detergent-treated membranes could be reconstituted in good yield into a translocation competent form once the detergent was removed.  相似文献   
70.
Comparative 16S rRNA sequencing was used to infer the phylogenetic relationships among selected species of mycobacteria and related organisms. The phylogeny inferred reflects the traditional classification, with major branches of the phylogenetic tree in general correspondence to the four Runyon groups and with numerical classification analyses. All the mycobacterial species compared, with the exception of M. chitae, are closely related (average similarity values greater than 95%). The slow growers form a coherent line of descent, distinct from the rapid growers, within which the overt pathogens are clustered. The distant relationship between M. chitae and the remaining mycobacteria suggests that this organism is incorrectly classified with the mycobacteria. M. paratuberculosis 18 was indistinguishable from M. avium-M. intracellulare-M. scrofulaceum serovar 1 by this analysis.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号